Remember seeing it before your very eyes in his 2000 blockbuster
Cast Away?
The 57-year-old actor had to lose 55lbs (nearly four stone) for the role of a shipwrecked plane-crash survivor. Extreme maybe, but the norm for many of our great method actors.

Aside from Oscar nominations and Hollywood credibility, scientists say there are some serious side effects. "Actors losing and gaining weight repeatedly, although admiral in showing the dedication to their craft, just isn't a good thing," explains expert nutritionist
Ian Marber
. "Dramatic changes in weight interrupts the metabolic rate. Even if you're losing, or gaining, at a less dramatic rate of say five to 10 pounds repeatedly, it still 'confuses' your metabolism," he warns.

He goes on to say: "if you're constantly introducing the idea of 'famine' - which is essentially what an extreme diet is - it creates fat stores that you can never reach." And while Marber can't confirm any direct links between Type 2 diabetes and dieting, what he can confirm is that it certainly raises the risk of diabetes. Plus he points out, yo-yo dieters always find weight management in their later years much tougher: "I suspect those actors who have lost weight and put it back on regularly will find it much harder to take off extra pounds when they're older," he adds.

A safe rate of weight loss, says Marber, is one to two pounds a week, "although in your first week of a diet you lose slightly more. Cutting into your glycogen stores for energy - which are held in water - means losing fluid too. Getting to the fat loss stage stakes slightly longer, but two pounds at a push is possible," he explains

Celebrity trainer and founder of
TwentyTwo Training
Dalton Wong agrees. "Try to lose anymore and I guarantee you won't win," he says.

On the physical side, says Wong, "dramatic weight loss means losing lean muscles mass too - the muscle you need to keep your body looking young and toned. On top of that depriving yourself of those vital nutrients and vitamins also leaves you with much weaker bones."

In the short term too you can tell if someone's going at it too extremely. "Women on extreme diets have that gaunt look in their faces - not to mention the dark circles because they can't sleep properly," adds Wong. "Besides, every time a woman diets her fat cells don't disappear, they shrink. All that happens when you start to eat normally again is that they get bigger much faster, so woman should never diet to the extremes - ever."

"Dieting to the Hollywood extreme, like Hanks did for Castaway, beats the hell out of your pancreas too - which means you become less efficient at managing your glucose levels and your weight. So it's no surprise that the acting elite tend to extreme diet under a doctor's supervision with all the vitamin shots they need - unless you're Christian Bale who just goes it alone," says Wong.

Also confirming our suspicions that Hollywood's jet set are defying the ageing process, scientists also say that such rapid weight loss should be rapidly ageing their faces. "Weight loss of over eight kilos (1.2 stone) in two months has been proving to noticeably age us," says plastic surgeon Rajiv Grover.

So just who else is putting their health on the line in Hollywood? Other losers include Christian Bale, who lost a startling 62 pounds on a diet of coffee and apples for his role as
The Machinist
in 2004. Reaching an eye watering 8.5 stone, he then returned as a stocky, ripped Batman in less than a year.

Christian Bale's emaciated version of his former self for The Machinist back in 2004.

Matthew McConaughey recently admitted he lived off a menu of Diet Coke, egg whites and a piece of chicken a day to get down to the frail frame of an AIDS patient in this year's
Dallas Buyers Club.
The American actor knocked three stone off his signature stocky shape.

Matthew McConaughey, the before ...and after.

Lest us not forget the gainers. Charlize Theron came away with an Oscar for her role as serial killer in
Monster
back in 2003. But with an extra two stone under her belt - lost rapidly for her acceptance speech later that year - we wonder if she also came away with some monstrous side effects?

And what about poor delicate Renée Zellweger, who has returned three times, and 28 pounds later, to her role as portly Bridgett Jones?